The most common industrial process for producing hydrogen chloride and sodium or potassium sulfate uses the reaction of sodium or potassium chloride with sulfuric acid at elevated temperatures (typically 500-840° C.) and is known as the Mannheim process (illustrated schematically in FIG. 1). Because of the high temperature at which the reaction is run, the Mannheim process is energy inefficient, using approximately half a barrel of oil per tonne of sulfate. Furthermore, the sulfate produced by the Mannheim process typically contains several percent of chloride impurities, while the HCl is emitted as a low-concentration (˜10%) mixture in combination with combustion gases. This low-concentration HCl can be converted to aqueous hydrochloric acid (typically 30-33% HCl) in an additional process.
Several attempts to provide improved versions of the Mannheim process are known. For example, a modified Mannheim process is disclosed in U.S. Pat. No. 4,342,737. In the invention disclosed in this patent, KCl is reacted with sulfuric acid at an equivalent ratio of 1.07-1.40 at temperatures from 250-500° C. While this process does provide sulfate containing less chlorine than that produced by the Mannheim process, it is still energy-intensive, requiring continuous heating to work.
U.S. Pat. No. 4,588,573 discloses an alternative approach to the production of potassium sulfate. This invention uses the wet reaction between KCl and H2SO4 to produce KHSO4; that is, the exothermicity of the reaction forming HCl is not sufficient to drive the formation of potassium sulfate (K2SO4) to completion. This invention further teaches dissolving the KHSO4 in water followed by precipitation to form a potassium hydrogen sulfate double salt, K3H(SO4)2, which represents three-fourths completion of the reaction to form K2SO4. The double salt is then filtered and again dissolved in water to form potassium sulfate. This process is not economical for industrial implementation, as it requires large quantities of water for the formation of the potassium sulfate, and large quantities of energy to evaporate the water in which the eventual product is dissolved.
U.S. Pat. No. 6,676,917 discloses a further process for production of HCl and neutralized sulfates. In this process, metal chloride reacts with sulfuric acid to yield a solution of KHSO4 in sulfuric acid. After evaporation of HCl, this solution is then neutralized either with NH3 to form K(NH4)SO4 or with potassium carbonate (K2CO3) to form K2SO4. This process is also impractical for commercial implementation, since it involves a tremendous waste of sulfuric acid, and the K2CO3 used as the neutralizing reagent is more expensive than the K2SO4 product of the process.
A fourth variation on the process is disclosed in U.S. Pat. No. 6,767,528. In the method disclosed in this patent, an approximately 1:1 mixture of metal chloride and H2SO4 is used to produce HCl. Reaction takes place in a rotary kiln kept at 550-600° C., and a counterflow between the gaseous components of the system and the solids is used to produce chloride-free sulfate. Again, the major disadvantage of this process is the large energy requirement. As with the Mannheim process, the HCl produced by this method is relatively impure, and in order to produce pure anhydrous HCl suitable for chlorination reactions, additional process steps are needed.
Finally, Greek Pat. No. 1003839 discloses a method for formation of K2SO4 via the reaction of a metal chloride with a mixture of concentrated H2SO4 and H2SO4·0.2SO3. The addition of H2SO4·0.2SO3 to the reaction mixture was performed to limit corrosion of the apparatus rather than for any benefit it would have on the reaction chemistry of the method. This method has several disadvantages as well. First of all, it is only adapted for use with KCl, and cannot be used with any other metal chloride as starting material (to produce the corresponding sulfate). Since the reaction mixture does not provide sufficient heat to sustain the reaction, the method also involves heating of the reactants, and hence suffers from the same energy inefficiency as the above-listed processes. Furthermore, the process does not produce K2SO4 directly (the product is obtained only after the aforementioned additional heating), nor does it produce a double salt.
Thus, there is a long-felt need for a process that is economical (including costs of start-up, production, and maintenance) and energy efficient for production of chloride-free metal sulfate and pure anhydrous HCl using a metal chloride salt as the starting material. The present invention discloses a method for doing so in which the metal chloride salt reacts with oleum in which the reactants are maintained at ambient temperature, and the supernatant liquid from the resulting sulfate salt solution is returned to the reactor, the exothermicity of the reaction between this liquid and the oleum being sufficient to drive the reaction without any need of additional heating; since there is no combustion, there are no combustion gases to dilute the discharged HCl.